The proposed research investigates the action of substituted quinones as inhibitors of in vivo chlorophyll fluorescence in plants. Such fluorescence inhibition reveals that quinones act as photosynthetic herbicides to inhibit the normal transfer of absorbed light to specialized domains where the process of photosynthesis is initiated. The site of action of these substituted quinones as fluorescence quenchers is not clearly known. The objective of the proposed project is to determine those structural and physicochemical parameters of substituted benzoquinones, naphthoquinones, and anthraquinones that enhance interaction with photosynthetic membranes. To examine quinone-membrane interactions, measurements of quinone-induced chlorophyll fluorescence quenching in chloroplasts will be made. Correlations between quinone structural properties and experimentally- measured chlorophyll fluorescence parameters will then be investigated. These correlations will determine the relation of chemical-physical properties to inhibitory action and aid in the elucidation of the operable mechanism of fluorescence quenching. The data will also enable the prediction of the inhibitory activity of extrinsic quinones in other biological systems. In particular, the investigation will yield an understanding of the chemical and physical properties of synthetic quinones that enhance their ability to inhibit the function of endogenous quinones which act as carriers in a diverse range of electron-transport chains.